Let's discuss how the action potential travels down the axon!
Since an axon can be unmyelinated or myelinated, the action potential has two methods to travel down the axon. We will refer to these methods as action potential conduction for unmyelinated axons, and saltatory conduction for myelinated axons. (Remember, conduction is making a pathway for an electric current to run; we can talk about conduction of ions or of an entire action potential).
Action potential conduction (in unmyelinated axons)
This is the type of conduction of action potentials that I described during our class this week. Basically, as long as the action potential occurs somewhere on the axon, it will run all the way down the axon. Here's a figure that shows this:
Follow the numbers down the figure. Basically, after the action potential is initiated at the axon hillock (and you still don't know exactly why), the electrical activity of the first action potential spreads (the positiveness of the AP spreads) and that triggers the nearby voltage-gated channels. Once triggered, another action potential occurs in that affected spot. That next action potential (#3) current spreads nearby and initiates another action potential (#5).
Therefore, in an unmyelinated axon, an action potential will be generated at every single spot all the way down the axon. Every little bit of axon has to be involved in the AP, since every little bit of the axon is affected by the current spread.
I have put this into an animation for you... I hope it helps you envision it!
Saltatory Conduction (in myelinated axons)
This manner of conduction is a lot faster because only the nodes of Ranvier are involved in action potential conduction. Here's the animation on this, and below it is the explanation:
I put the animation above so that you could view it at the same time and in the same window as the unmyelinated axon action potential conductance. Can you see which is faster? I hope you can tell it is the myelinated one.
In order for an action potential to occur, you saw that sodium and potassium ions have to move across the axonal membrane. Remember? Well, wherever the Schwann cells (in yellow in the animation) wrap around the axon, the sodium and potassium ions cannot cross the membrane; the Schwann cells wrap too tightly around the axonal membrane for there to be any extracellular space underneath them. Therefore, the only place that an action potential can occur is at the node of Ranvier-- the space between the Schwann cells. Because of this, the action potential seems to jump from node to node along the axon. "Jumping" is what the word "saltatory" means.
So, saltatory conduction is when the action potential jumps down the axon from node to node.
Why demyelinating diseases stop axons from working
Did you notice that the current spread of the action potential is farther in the myelinated axon than in the unmyelinated axon? Why? Because the myelin insulates the axon and allows the current to spread farther before it runs out.
Think about the myelinated axon now. Knowing that it takes work on the neuron's part to make the gated channel proteins, I hope you can understand that it would be a waste of energy for the neuron to put gated channels underneath the myelin, since they could never be used. So, myelinated axons only have gated channels at their nodes.
In a demyelinating disease, the myelin sheath decays... the Schwann cells die selectively. When this happens, and the myelin sheath is gone, the current from the initial action potential cannot spread far enough to affect the region of the axon where the gated channels are found. So, conductance of the action potential stops. And the axon is never able to send its output (the action potential) to its axonal terminals. If this axon innervated muscle, that muscle can no longer be controlled.
This is a terrible condition. We may have more opportunities to discuss this.
© 2011 STCC Foundation Press